Movable -Bed Modeling Guidance 



52. In general, most researchers agree that two approaches/concepts are 

 important in modeling how particles are moved from one bed location to 

 another: 



a. Fall velocity similarity. 



b . Incipient motion similarity. 



Recent studies (Hughes and Fowler 1990) indicate that the fall speed scaling 

 guidance produces good results for energetic situations such as occur in the 

 surf zone where turbulent energy associated with breaking waves dominates. 

 Scaling by incipient motion criteria is more appropriate in situations where 

 sediment transport is predominantly by bed load. Since the overwhelming 

 majority of sediment transport for these tests was by suspended load, the fall 

 speed guidance was used. Appropriate fall speed scaling criteria are: 



Fall 



Speed Scaling Guidance for Wave-Energv-Dominated Erosion 



1) 



Fall speed parameter (H/wT) similarity. 



2) 



Time scaled by Froude (Fr = V/(gi) % ) similarity. 



3) 



Model is undistorted (N, = N x = N y = N z ) . 



4) 



Use fine sand (d = 0.08mm lower limit) as model sediment 

 at largest possible scale ratio. 



For the above items: 



V = an appropriate velocity 



!L = characteristic length 



N = scale ratio (prototype to model) 

 Subscripts i, x, y, and z are characteristic length, length in the x 

 direction, length in the y direction, and length in the z direction, 

 respectively. Similarity between model and prototype fall speed parameters is 

 achieved when 



h£fl = h£fl < 62 > 



l U 1 \model I W 1 Iprototype 



which, for an undistorted model, reduces to 



50 



